Catadioptric reduction projection optical system and method

ABSTRACT

A catadioptric reduction projection optical system having a first lens unit having negative refractive power and widening a light beam from a reticle, a prism type beam splitter for transmitting therethrough a light beam from the first lens unit, a concave reflecting mirror for returning the light beam emerging from the beam splitter to the beam splitter while converging it, and a second lens unit having positive refractive power and converging the light beam returned to the beam splitter and reflected by the beam splitter, and forming the reduced image of a pattern on the reticle on a wafer.

This is a continuation of application Ser. No. 08/482,505 filed Jun. 7,1995, which is a continuation of application Ser. No. 08/062,725 filedMay, 18, 1993, which is a continuation of application Ser. No.07/948,248 filed Sep. 21, 1992, all now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a catadioptric reduction projection opticaland method for use, for example, in an exposure apparatus for themanufacture of semiconductive elements, and particularly suitable forapplication to an optical system for reduction-projecting a pattern moreenlarged than the pattern of a real element.

2. Related Background Art

Semiconductive integrated circuits have become more and more minute andexposure apparatuses for printing the patterns thereof are required tohave higher resolving power. To satisfy this requirement, the wavelengthof a light source must be made short and the numerical aperture (N.A.)of an optical system must be made great. However, if the wavelengthbecomes short, glass materials standing practical use will becomelimited because of the absorption of light. If the wavelength becomes300 mm or shorter, what can be practically used will be only syntheticquartz and fluorite (calcium fluoride). Also, fluorite is bad intemperature characteristic and cannot be used in a great quantity.Therefore, it is very difficult to make a projection lens of arefracting system alone. Further, because of the difficulty ofaberration correction, it is also difficult to make a projection opticalsystem having a great numerical aperture of a reflecting system alone.

So, there have been proposed various techniques of constructing aprojection optical system by combining a reflecting system and arefracting system. An example of such techniques is a ring field opticalsystem as disclosed in U.S. Pat. No. 4,747,678. In this optical system,an off-axis light beam is used so that incident light and reflectedlight may not interfere with each other, and the design is made suchthat only an off-axis zonal portion is exposed to light.

As another example, a projection exposure apparatus comprising aprojection optical system having a beam splitter disposed therein, and acatadioptric system for collectively projecting the image of a reticle(mask) by an on-axis light beam is disclosed, for example, in U.S. Pat.Nos. 3,698,808 and 4,953,960.

FIG. 5 of the accompanying drawings schematically shows the opticalsystem disclosed in U.S. Pat. No. 4,953,960. In FIG. 5, a light beamfrom a reticle 21 on which a pattern to be reduction-transferred isdepicted is converted into a substantially parallel light beam by a lensunit 22 having positive refractive power and is applied to a prism typebeam splitter (beam splitter cube) 23. The light beam transmittedthrough the joint surface 23a of this beam splitter 23 is diffused by acorrection lens unit 24 having negative refractive power and isreflected by a concave reflecting mirror 25. The light beam reflected bythe concave reflecting mirror 25 passes through the correction lens unit24 again and is reflected by the joint surface 23a of the beam splitter23, whereafter it is converged on a wafer 27 by a lens unit 26 havingpositive refractive power, and the reduced image of the reticle patternis formed on the wafer 27. An example in which a half mirror comprisinga plane parallel plate is used instead of the prism type beam splitter23 is also disclosed.

In the example of the prior art shown in FIG. 5, the reductionmagnification of the entire system is 1/4 and the magnification in theconcave reflecting mirror 25 is 0.287. Also, the magnification of theconcave reflecting mirror in the example of the construction using theobliquely disposed plane parallel plate having a half-transmittingsurface is 0.136. That is, in the example of the prior art, the designis made such that with the burden of reduction magnification cast on theconcave reflecting mirror, aberrations attributable to a concavereflecting mirror of small reduction magnification are corrected by thecorrection lens unit 24 and the lens unit 26.

In the ring field optical system according to the prior art, however, itis difficult to make the numerical aperture great. Moreover, it is alsoimpossible to expose collectively and therefore, it is necessary toeffect exposure while moving the reticle and the wafer at differentspeeds correspondingly to the reduction ratio of the optical system, andthis has led to the inconvenience that the construction of themechanical system becomes complex.

Also, in the construction disclosed in U.S. Pat. No. 3,698,808, there isthe inconvenience that the flare by the reflection on the refractingsurface of the optical system subsequent to the beam splitter is great.Further, characteristics such as the reflectance irregularity,absorption and phase change of the beam splitter are not at all takeninto account and therefore, the resolving power is low and themagnification of the entire system is one-to-one magnification, and theapparatus of the prior art cannot possibly stand the use as asemiconductor manufacturing exposure apparatus of the coming generationof which higher resolving power is required.

Furthermore, in the projection optical system disclosed in U.S. Pat. No.4,953,960, almost all of the reduction magnification of the entiresystem is borne by the concave reflecting mirror, and this leads to theinconvenience that spherical aberration created by the concavereflecting mirror is great. Accordingly, an optical system forcorrecting that spherical aberration becomes complicated. Also, sincethe design is made such that the light beam from the reticle 21 isconverted into a substantially parallel light beam by the lens unit 22of positive refractive power, the spacing between the reticle 21 and thebeam splitter 23 becomes long, and this leads to the bulkiness of theoptical system.

SUMMARY OF THE INVENTION

In view of the above-noted points, the present invention has as anobject the provision of a reduction projection optical system of aconstruction in which a beam splitter is disposed in a catadioptricsystem and in which spherical aberration attributable to a concavereflecting mirror is small.

A further object of the present invention is to provide an improvedexposing method in which a reticle is illuminated with a linearlypolarized light beam.

The catadioptric reduction projection optical system according to thepresent invention is an optical system for reduction-projecting thepattern of a first surface (1) onto a second surface (5), as shown, forexample, in FIG. 1 of the accompanying drawings, and has a first lensunit G1 preferable having reduction magnification and widening a lightbeam from the first surface, a prism type beam splitter (2) transmittingtherethrough or reflecting the light beam from the first lens unit, aconcave reflecting mirror (4) returning the light beam emerging from thebeam splitter to the beam splitter while converging the light beam, anda second lens unit G2 having positive refractive power and convergingthe light beam returned to the beam splitter and reflected by ortransmitted through the beam splitter, and forming the reduced image ofthe pattern of the first surface (1) on the second surface (5).

In this case, it is preferable that the radius of curvature of theconcave reflecting mirror (4) be set to a range seventeen times totwenty-five times as great as the diameter of the exposure area (imagecircle) on the second surface (5).

Also, it is preferable that the magnification of the concave reflectingmirror (4) be 0.6 time to 1.1 times.

In addition, it is preferable that the inclination of the off-axisprincipal ray incident on the concave reflecting mirror (4) with respectto the optical axis be 5 degrees or less.

According to the present invention, the image of the pattern of thefirst surface reduced by the first lens unit of having reductionmagnification is located near the conjugate point of the center ofcurvature of the concave reflecting mirror and therefore, the concavereflecting mirror can be used at substantially one-to-one magnification.This leads to the advantage that spherical aberration attributable tothe concave reflecting mirror can be decreased and as a whole,aberrations can be corrected well.

Also, there is the advantage that when the radius of curvature of theconcave reflecting mirror is seventeen times to twenty-five times asgreat as the diameter of the exposure area of the second surface,astigmatism and distortion can be corrected easily and a predeterminedreduction magnification can be obtained easily.

Further, when the magnification of the concave reflecting mirror is 0.6time to 1.1 times, a predetermined reduction magnification is obtainedas a whole, and then the spherical aberration by the concave reflectingmirror can be corrected best.

Furthermore, there is the advantage that when the inclination of theoff-axis principal ray incident on the concave reflecting mirror withrespect to the optical axis is limited to 5° or less, the amount ofaberration such as astigmatism can be suppressed within a predeterminedrange and the irregularity of the reflectance and transmittance in thebeam splitter can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the basic construction of anembodiment of a catadioptric reduction projection optical systemaccording to the present invention.

FIG. 2 is a lens construction view showing the specific construction ofthe optical system of FIG. 1.

FIGS. 3A, 3B, 3C and 3D show the longitudinal aberrations of theembodiment of FIG. 2.

FIGS. 4A, 4B, 4C and 4D show the lateral aberrations of the embodimentof FIG. 2.

FIG. 5 is a cross-sectional view showing the basic construction of acatadioptric reduction projection optical system according to the priorart.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, in a construction comprising acombination of a reflecting system and a refracting system, an on-axislight beam is used to expose a wide area collectively. Also, since thereis no chromatic aberration in the reflecting system, the concavereflecting mirror (4) is endowed with most of the refractive power ofthe entire system to suppress the creation of chromatic aberration.Also, for the suppression of spherical aberration in the concavereflecting mirror (4) which is a main purpose of the present invention,the light beam incident on the concave reflecting mirror (4) can becomesubstantially perpendicular to the reflecting surface thereof. Thismeans that the concave reflecting mirror (4) can be used substantiallyas one-to-one magnification imaging.

The simplest construction for that is a construction in which the firstsurface (1) is located near the center of curvature of the reflectingsurface of the concave reflecting mirror (4) (actually the conjugatepoint thereof by the beam splitter (2)). However, if the first surface(1) becomes too close to the concave reflecting mirror (4), the maximumvalue of the inclination of the off-axis principal ray with respect tothe optical axis will become great and astigmatism, etc. will becomegreat. Further, if the angle of incidence of the light beam onto theprism type beam splitter (2) becomes great, the loss of the quantity oflight will become great and flare and imaging performance will bedeteriorated and thus, good imaging will not be accomplished.

So, in the present invention, the light beam from the first surface (1)is widened by the first lens unit G1 preferably having reductionmagnification, whereby the reduced image of the pattern on the firstsurface (1) is disposed near the conjugate point of the center ofcurvature of the concave reflecting mirror (4). Since the whole of theoptical system must be made into a reduction system, the reduction ofthe image by the first lens unit G1 is useful. Further, the image isreduced also by the second lens unit G2 having positive refractive powerwhich is disposed between the beam splitter (2) and the second surface(5), whereby a desired reduction rate can be obtained as a whole, inspite of the concave reflecting mirror (4) being of substantiallyone-to-one magnification.

Also, the separation of the incident light and the reflected light fromeach other is effected by the prism type beam splitter (2). The use ofthe prism type beam splitter is for preventing the creation ofastigmatism and coma caused by the use of an obliquely disposed planeparallel plate having a half-transmitting surface. Further, to make theangular characteristic of the beam splitter (2) good, it is preferablethat for example, the number of layers of multilayer film used astranslucent film be made as small as possible. More specifically, it ispreferable that transmittance>50%>reflectance. Further, where use ismade of multilayer film which has a considerably strong polarizingcharacteristic, a quarter wavelength plate (3) is disposed between thebeam splitter (2) and the concave reflecting mirror (4), wherebyreflection efficiency and transmission efficiency in the joint surfaceof the beam splitter (2) can be improved greatly. Accordingly, theeffective utilization of the quantity of light and a reduction in flarecan be achieved.

A description will now be given of the reason why the radius ofcurvature of the concave reflecting mirror (4) should preferably beseventeen times to twenty-five times as great as the diameter of theexposure area (image circle) on the second surface (5). In a concavereflecting mirror, a certain degree of reduction magnification can beachieved by the converging function thereof and influence is imparted toPetzval sum, astigmatism and distortion and thus, it becomes possible tomaintain the aberration balance with the refracting system comprisingthe first lens unit G1 and the second lens unit G2 good. That is, if theradius of curvature of the concave reflecting mirror (4) is belowseventeen times the diameter of the image circle of the second surface(5), it will be advantageous for the correction of chromatic aberration,but Petzval sum will increase in the positive direction and astigmatismand distortion will also increase.

The reason is that if the radius of curvature of the concave reflectingmirror becomes small and the refractive power thereof becomes great, thereduction magnification of the first lens unit G1 also becomes great inorder that the light beam passing through the beam splitter (2) beforeand after the reflection on the concave reflecting mirror (4) may bemade substantially perpendicular to the reflecting surface of theconcave reflecting mirror and therefore, it is necessary that therefractive power of the positive refractive power of the second lensunit G2 be made great for the correction of spherical aberration.However, the second lens unit G2 is disposed near the second surface (5)as the image plane and therefore, for the correction of aberrations,refractive power greater than the refractive power of the first lensunit G1 is required of the second lens unit and thus, Petzval sumincreases remarkably. Accordingly, to correct aberrations better, it isdesirable that the radius of curvature of the concave reflecting mirror(4) be greater than nineteen times the diameter of the image circle ofthe reduced image.

If conversely, the radius of curvature of the concave reflecting mirror(4) becomes great beyond twenty-five times the diameter of the imagecircle of the reduced image, it will be advantageous for the correctionof astigmatism and distortion, but it will become difficult to obtain adesired reduction magnification and the correction of chromaticaberration will become insufficient, and this is not very practical.

In the present invention, the concave reflecting mirror (4) is used atsubstantially one-to-one magnification, and it is preferable that therange of the magnification thereof be 0.6 time to 1.1 times. That is, ifthe magnification is smaller than 0.6 time, spherical aberration willbecome great and the optical system for correcting it will becomecomplicated. On the other hand, now that the magnification of the entiresystem is a reduction magnification, it is originally not preferablethat the magnification of the concave reflecting mirror (4) exceed 1time, but the fact that the magnification becomes great means that theradius of curvature becomes great and further that spherical aberrationcan be made small. So, it is considered that when importance is attachedto an improvement in aberrations even at the sacrifice of magnification,up to the order of 1.1 times, can be allowed as the magnification of theconcave reflecting mirror (4).

A description will now be given of the reason why it is preferable thatthe inclination of the off-axis principal ray incident on the concavereflecting mirror (4) with respect to the optical axis be 5 degrees orless. First, unless the inclination of the off-axis principal ray islimited like this, astigmatism, etc. on the concave reflecting mirror(4) will become too great. Further, the inclination of the off-axisprincipal ray incident on the concave reflecting mirror (4) is equal tothe inclination of the off-axis principal ray incident on the beamsplitter (2) with respect to the optical axis. If the inclination of theoff-axis principal ray with respect to the beam splitter (2) is limitedlike that, the irregularity of reflectance and transmittance on thejoint surface of the beam splitter (2) will become small and theirregularity of the variation in phase will also become small andtherefore, the imaging performance will be improved as a whole.

A specific embodiment of the catadioptric reduction projection opticalsystem according to the present invention will hereinafter be describedwith reference to FIG. 1 to FIGS. 4A, 4B, 4C and 4D. This embodiment isone in which the present invention is applied to the optical system ofan exposure apparatus for the manufacture of semiconductors in which thewavelength use is 248 nm and the reduction magnification is 1/5.

Referring to FIG. 1 which schematically shows the construction of theoptical system of the present embodiment, the reference numeral 1designates a reticle on which a pattern for an integrated circuit isformed. A first lens unit G1 having reduction magnification, a prismtype beam splitter 2, a quarter wavelength plate 3 and a concavereflecting mirror 4 are successively disposed along the optical axisperpendicular to the reticle 1, and a second lens unit G2 havingpositive refractive power and a wafer 5 are successively disposed in adirection in which the reflected light by the concave reflecting mirror4 is reflected by the joint surface 2a of the beam splitter 2.

The reticle 1 is illuminated by an illuminating optical system, notshown, and a light beam emerging from the reticle 1 is caused to divergeby the first lens unit G1 having reduction magnification and enter thebeam splitter 2, and the light beam transmitted through the jointsurface 2a of this beam splitter 2 is caused to be incident on theconcave reflecting mirror 4 through the quarter wavelength plate 3. Theradius of curvature of the concave reflecting mirror 4 is about 400 mm.The light beam reflected by the concave reflecting mirror 4 passesthrough the quarter wavelength plate 3 while converging and again entersthe beam splitter 2, and the light beam reflected by the joint surface2a of this beam splitter 2 is condensed on the wafer 5 by the secondlens unit G2. Thereby the reduced image of the pattern on the reticle 1is formed on the wafer 5.

Also, a light beam polarized in parallelism to the plane of the drawingsheet of FIG. 1 (P-polarized light) is used as illuminating light. Inthis case, most light is transmitted through the joint surface 2a due tothe polarization characteristic of the beam splitter 2, and thistransmitted light is further transmitted through the quarter wavelengthplate 3, whereby it becomes circularly polarized light. The beam of thiscircularly polarized light is reflected by the concave reflecting mirror4 and becomes circularly polarized light opposite in direction, but whenthe beam of this circularly polarized light opposite in direction isagain transmitted through the quarter wavelength plate 3, it becomeslinearly polarized light perpendicular to the plane of the drawing sheetof FIG. 1. Most of the light beam polarized in a direction perpendicularto the plane of the drawing sheet of FIG. 1 by the polarizationcharacteristic of the beam splitter 2 is reflected by the joint surface2a and travels toward the wafer 5. Thereby, the return light to thereticle 1 is decreased and thus, the effective utilization of the lightbeam and a decrease in flare are achieved.

Further, it is desirable that single-axis crystal (for example, rockcrystal) of a small thickness be used as the quarter wavelength plate 3.The reason is that if the light beam transmitted through the quarterwavelength plate deviates from a parallel light beam, astigmatism willoccur for abnormal rays. This astigmatism cannot be corrected by amethod of cementing two crystals together by rotating the optical axesthereof by 90° relative to each other as is usually done for awavelength plate. That is, astigmatism will occur for both of normalrays and abnormal rays.

Assuming that the amount of this astigmatism is represented by wavesurface aberration W, the wave surface aberration is expressed by thefollowing equation:

    W=(n.sub.o -n.sub.e)dθ.sup.2 /2,

where (n_(o) -n_(e)) is the difference between the refractive indices ofnormal rays and abnormal rays, d is the thickness of the crystal, and θis the deviation from the parallel light beam, i.e., the angle ofdivergence (or convergence) of the light beam.

For example, where the quarter wavelength plate is constructed by rockcrystal, (n_(o) -n_(e))=0.01 and the divergent (convergent) state of thelight beam is θ=14°. When the wavelength used is λ, to maintain asufficiently good imaging performance, it is necessary to maintain thewave surface aberration W less than a quarter wavelength, i.e., λ/4. Forthat purpose, on the assumption that the wavelength λ is 248 nm, fromthe foregoing equation, d must be

    d<100 μm.

The quarter wavelength plate 3 is very thin like this and therefore, itmay be adhesively secured to and supported by the beam splitter 2.

In the construction of FIG. 1, the design is made such that the lightbeam transmitted through the beam splitter 2 is directed to the concavereflecting mirror 4 and the light beam reflected from this concavereflecting mirror 4 and further reflected by the beam splitter 2 isconverged by the second lens unit G2. However, the concave reflectingmirror 4 may be disposed so as to sandwich the beam splitter 2 betweenit and the second lens unit G2, and the light beam reflected by the beamsplitter 2 may be applied to the concave reflecting mirror 4, and thelight beam reflected from this concave reflecting mirror 4 andtransmitted through the beam splitter 2 may be converged by the secondlens unit G2. Also, if a polarizing beam splitter is used as the beamsplitter 2, reflectance and transmittance can be further improved by thecombination thereof with the quarter wavelength plate. However, even ifthe beam splitter 2 is not a polarizing beam splitter but an ordinarybeam splitter, it has some degree of polarizing characteristic andtherefore, reflectance and transmittance can be improved by thecombination thereof with the quarter wavelength plate.

A specific example of the construction of the optical system of FIG. 1will hereinafter be described.

FIG. 2 shows a specific lens construction view of such optical system.As shown in FIG. 2, the first lens unit G1 comprises, in succession fromthe reticle 1 side, a negative meniscus lens L₁₁ having its convexsurface facing the reticle 1, a biconvex lens L₁₂, a biconvex lens L₁₃,a negative meniscus lens L₁₄ having its convex surface facing thereticle 1, and a biconcave lens L₁₅. The second lens unit G2 comprises,in succession from the prism type beam splitter 2 side, a biconvex lensL₂₁, a biconcave lens L₂₂, a biconvex lens L₂₃, a negative meniscus lensL₂₄ having its convex surface facing the beam splitter 2, and a positivemeniscus lens L₂₅ having its convex surface facing the beam splitter 2.The quarter wavelength plate 3 in FIG. 1 is not shown in FIG. 2 becauseits thickness is negligibly small.

In order to represent the shapes of and the spacings between the lensesof FIG. 2, with the reticle 1 as the first surface, the surfaces throughwhich the light emerging from the reticle 1 passes until it arrives atthe wafer 5 are called the ith surface (i=2, 3, . . . , 27). As regardsthe sign of the radius of curvature r_(i) of the ith surface, betweenthe reticle 1 and the concave reflecting mirror 4, a case where the ithsurface is convex relative to the reticle 1 is chosen to positive, andbetween the joint surface of the beam splitter and the wafer, a casewhere the ith surface is convex relative to the joint surface is chosento positive. Also, the sign of the surface spacing d_(i) between the ithsurface and the (i+1)th surface is chosen to negative in the areawherein the reflected light from the concave reflecting mirror 4 passesto the joint surface of the beam splitter 2, and is chosen to positivein the other areas. The radius of curvature r_(i), the surface spacingd_(i) and the glass materials of FIG. 2 will be shown in Table 1 below.In the column of glass materials CaF₂ represents fluorite and SiO₂represents quartz glass. The refractive indices of quartz glass andfluorite for the standard wavelength used (248 nm) are as follows:

quartz glass: 1.50855

fluorite: 1.46799

                  TABLE 1                                                         ______________________________________                                                                          glass                                       i                ri                     material                              ______________________________________                                                  ∞   161.900                                                   2               473.382                                                                                    23.000                                                                                  CaF.sub.2                              3               171.144                                                                                     6.000                                           4               172.453                                                                                    29.000                                                                                  SiO.sub.2                              5              -246.006                                                                                 16.818                                              6               148.803                                                                                    20.000                                                                                  SiO.sub.2                              7              -2656.033                                                                                1.000                                               8               230.632                                                                                    16.000                                                                                  CaF.sub.2                              9               102.960                                                                                    30.000                                           10            -143.364                                                                                  18.000       SiO.sub.2                              11             147.730                                                                                     223.179                                          12            ∞                                                                                         145.000                                                                             SiO.sub.2                               13             ∞                                                                                        20.000                                        14            -394.591                                                                                 -20.000                                              15            ∞                                                                                         -72.500                                                                          SiO.sub.2                                  16             ∞                                                                                        72.500                                                                               SiO.sub.2                              17             ∞                                                                                        42.626                                        18             81.489                                                                                      17.000                                                                                  CaF.sub.2                              19             -1339.728                                                                               7.000                                                20             -172.194                                                                                11.000        SiO.sub.2                              21             204.909                                                                                      4.300                                           22             461.579                                                                                     23.800                                                                                  CaF.sub.2                              23             -142.095                                                                                 0.200                                               24              55.322                                                                                     18.273                                                                                  SiO.sub.2                              25              40.925                                                                                      3.000                                           26              53.590                                                                                     11.000                                                                                  CaF.sub.2                              27             849.726                                                                                     17.541                                           ______________________________________                                    

In the embodiment of FIG. 2, the reduction magnification is 1/5, thenumerical aperture is 0.45, and the diameter d of the effective exposurearea (image circle) on the wafer 5 is 20 mm. The radius of curvature rof the concave reflecting mirror 4 is 394.591 mm, and the radius ofcurvature r is about 19.7 times as great as the diameter d. Also, themagnification β in the concave reflecting mirror 4 is about 0.707 and iswithin said range, and can be regarded as substantially one-to-onemagnification.

Further, the maximum value of the inclination of the marginal ray (Randray) from the on-axis object point incident on the concave reflectingmirror 4 with respect to the optical axis is 7.72°, and the maximumvalue of the inclination of the off-axis principal ray incident on theconcave reflecting mirror 4 with respect to the optical axis is 3.23°.Incidentally, the maximum value of the inclination of the Rand rayemerging from the concave reflecting mirror 4 with respect to theoptical axis is 10.76°.

Longitudinal aberration graphs of the embodiment of FIG. 2 are shown inFIGS. 3A-3D, and lateral aberration graphs of the same embodiment areshown in FIGS. 4A-4D. In these aberration graphs, curves J, P and Q showthat the wavelengths used are 248.4 nm, 247.9 nm and 248.9 nm,respectively. From these aberration graphs, it is seen that in thepresent embodiment, in spite of the numerical aperture being great,aberrations including chromatic aberration are corrected well in thearea of a wide image circle of a radius 10.6 mm.

Finally, flare will be described for information. The concave reflectingmirror 4 in the above-described embodiment is used at substantiallyone-to-one magnification and therefore, light reflected by the concavereflecting mirror 4 tends to return. Accordingly, ghost (i.e., flare)which is the inverted image of the original image is liable to becreated on the surfaces of the reticle 1 and the wafer 5. This isreduced by the quarter wavelength plate 3, but the reduction may beinsufficient when limitations to flare are severe. However, whenlimitations to flare are severe, it can be coped with by covering oneside of the reticle 1 or one side of the wafer 5 to thereby eliminateghost. This method is suited for a slit scan type exposure apparatus.

Of course, the present invention is not restricted to theabove-described embodiment, but can adopt various constructions withoutdeparting from the gist of the invention.

What is claimed is:
 1. A catadioptric optical reduction system forforming a pattern of an object onto a substrate, having an object spacenumerical aperture, from the long conjugate end to the short conjugateend, comprising:first lens means having an emerging numerical aperture,the emerging numerical aperture being larger than the object spacenumerical aperture; a beam splitter; a concave mirror; a quarter-waveplate placed between said beam splitter and said concave mirror; andsecond lens means for providing a positive power, wherein the power ofsaid first lens means provides only enough power to image an entrancepupil of the system at infinity to an aperture stop near said mirror,and the positive power of said second lens means images the exit pupilof the system to infinity.
 2. An optical reduction system as in claim 1,wherein:said first lens means and said second lens means includerefractive elements made of at least two different materials.
 3. Anoptical reduction system as in claim 2, wherein said beam splitter is acube.
 4. An optical reduction system as in claim 2, wherein a positionfor said aperture stop is between a portion of said beam splitter andsaid concave mirror.
 5. An optical reduction system as in claim 1,wherein said beam splitter is a cube.
 6. An optical reduction system asin claim 1, wherein a space between said beam splitter and said concavemirror has no lens element.
 7. An optical reduction system as in claim1, wherein a position for said aperture stop is between a portion ofsaid beam splitter and said concave mirror.
 8. An optical reductionsystem as in claim 1, wherein said concave mirror has substantiallyone-to-one magnification.
 9. A method for forming a reduced image of apattern of a first object onto a second object, said methodcomprising:providing a catadioptric optical reduction system having afirst object space numerical aperture; illuminating said first object;forming said reduced image onto said second object with saidcatadioptric optical reduction system, said catadioptric opticalreduction system, from the long conjugate end to the short conjugateend, comprising:first lens means having an emerging numerical aperture,the emerging numerical aperture being larger than the object spacenumerical aperture; a beam splitter; a concave mirror; a quarter-waveplate placed between said beam splitter and said concave mirror; andsecond lens means for providing a positive power, wherein the power ofsaid first lens means provides only enough power to image an entrancepupil of the system at infinity to an aperture stop near said mirror,and the positive power of said second lens means images the exit pupilof the system to infinity.
 10. A method as in claim 9, wherein:saidfirst lens means and said second lens means include refractive elementsmade of at least two different materials.
 11. A method as in claim 10,wherein said beam splitter is a cube.
 12. A method as in claim 10,wherein the power of the first lens means is sufficient to image asystem entrance pupil at infinity between a portion of said beamsplitter and said concave mirror.
 13. A method as in claim 9, whereinsaid beam splitter is a cube.
 14. A method as in claim 9, wherein aspace between said beam splitter and said concave mirror has no lenselement.
 15. A method as in claim 9, wherein the power of the first lensmeans is sufficient to image a system entrance pupil at infinity betweena portion of said beam splitter and said concave mirror.
 16. A method asin claim 9, wherein said concave mirror has substantially one-to-onemagnification.